With the long-range goal of gaining a general understanding of the psychophysical process -- initial transformations of stimuli into underlying sensory components, interactions of these components, and resulting overt responses -- the proposed research examines similarities in sensory/perceptual responses to stimuli of different modalities. Based on the theory of the "unity of the senses," which states that sensory systems display important communalities, the proposed research uses psychophysical methods to examine similarities across touch, hearing, vision, and taste. The first series of studies examines structural similarities in attributes of perceptual experience (e.g., intensity) and spatio- temporal features (e.g., temporal patterns), using two methods: cross-modal and intramodal scaling, in which subjects rate the similarity/dissimilarity of, for example, tactile to tactile or auditory to gustatory sensations, and free cross-modal matching, where subjects match sensations of different modalities. The second series examines how structural similarities across the senses reveal themselves in functional tasks by measuring accuracy and speed of response in discriminating stimuli of one modality, given simultaneous simulation with "matching" or "mismatching" stimuli from another modality. Experiments will determine whether the cross-modal interactions take place at a perceptual or semantic locus. The third series uses scaling methods such as magnitude estimation to evaluate, in individual subjects, evidence for processes of linear summation of perceived intensity common to hearing (binaural summation, summation of widely separated sound frequencies) and touch (summation of widely separated vibration frequencies). The fourth series uses scaling methods to evaluate mechanisms of absolute vs relative psychophysical judgement common to different senses, and the role of intramodal and cross-modal similarity in determining how absolute such judgements are. In sum, the proposed research constitutes a multifaceted and broad attack on fundamental issues of sensory perception and judgement.